HFC-125 is a useful compound which can be a substitute for flon which does not contain chlorine, and it is used as a refrigeration medium, a forming agent, a propellant and so on. Fluorination of tetrachloroethylene is employed as a process of producing HFC-125. In such a process, dichlorotetrafluoroethane, dichlorotrifluoroethane, hexafluoroethane, CFC-115 and so on are produced as by-products.
Among those, CFC-115 has a boiling point of -38.7.degree. C. which is considerably close to a boiling point of HFC-125 (-48.5.degree. C.) as an objective product. Further, a relative volatility between these two compounds is near one. Particularly when a mixture contains HFC-125 at a concentration of not less than 95 mol % (thus, CFC-115 is not more than 5 mol %), the relative volatility is about 1.04. Therefore, it requires a distillation apparatus having many plates to separate HFC-125 in its higher concentration out of such a mixture using a conventional distillation operation, which generally means that the separation using the distillation is extremely difficult.
In the present specification, the relative volatility (.alpha.) is defined as follows when a solution consisting essentially of at least two components A and B in question (a boiling point of component A&lt;a boiling point of component B) is in a vapor-liquid equilibrium state: EQU .alpha.=(y.sub.A /x.sub.A)/(y.sub.B /x.sub.B)
wherein x.sub.A is a molar fraction of lower boiling component A in the liquid phase, x.sub.B is a molar fraction of higher boiling component B in the liquid phase, y.sub.A is a molar fraction of lower boiling component A in the vapor phase which is equilibrated with the liquid phase and y.sub.B is a molar fraction of the higher boiling component B in such a phase.
Extractive distillation has been employed as a process of separating one component from a mixture of a system of which relative volatility is close to one. With regard to separation of a mixture of HFC-125 and CFC-115, for example U.S. Pat. No. 5,087,329 discloses an extractive distillation process which uses a fluorocarbon containing 1 to 4 carbon atoms as an extractant.
In the process disclosed in U.S. Pat. No. 5,087,329, a relative volatility between HFC-125 and CFC-115 is about 1.2 based on calculation using figures shown in Example 1 thereof. Thus, in order that a concentration of HFC-125 is increased from HFC-125/CFC-115=7/93 (mol %/mol %) to HFC-125/CFC-115=99.7/0.3 (mol %/mol %) using distillation, about 40 theoretical plates are required. The number of the theoretical plates were calculated as explained below.